LTE: Carrier Aggregation based ICIC

In a Heterogeneous network (HetNet), terminals in Cell Range Extension (CRE), zone would experience severe interference from the aggressor cells. Aggressor cell could be a macro cell in case of macro-pico or a femto cell in case of macro-femto scenario. A number of features added to the 3GPP LTE specifications to mitigate the above mentioned interference problem in HetNets with small cells. Inter-cell interference coordination (ICIC) has the task to manage radio resources such that inter-cell interference is kept under control.

ICIC is discussed in detail in the earlier post. It is introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. Moreover, only frequency domain ICIC was prioritized which manages PRBs, such that multiple cells coordinate use of frequency domain resources. The major problem here is the interference introduced by downlink control channels.

The enhanced ICIC (eICIC) is introduced in 3GPP LTE Release-10 to deal with interference issues in HetNets, and mitigate interference on traffic and control channels. The major change in eICIC is the addition of time domain ICIC. Time domain ICIC is realized through the use of Almost Blank Subframes (ABS). For the detailed discussion, check the post eICIC.

Another approach is based on carrier aggregation (CA) with cross-carrier scheduling which is mainly frequency domain ICIC. The main difference as compared to frequency domain ICIC introduced in Release-8 is that the CA based ICIC would work with control channels (PCFICH, PHICH, and PDCCH) as well.

Carrier Aggregation based ICIC

Carrier aggregation (CA) is one of the most important LTE-Advanced features introduced in Release-10. With CA, two or more component carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100MHz. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities.

When CA is configured, the UE only has one RRC connection with the network. The serving cell managing the UE’s RRC connection is referred to as the Primary Cell (PCell). Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells.

In CA, a UE may be scheduled via PDCCH over multiple serving cells simultaneously. Cross-carrier scheduling with the Carrier Indicator Field (CIF) allows the PDCCH of a serving cell to schedule resources on another serving cell. In other words, a UE receiving a downlink assignment on one CC may receive associated data on another CC.

Cross-carrier scheduling is an important feature in HetNets where inter-cell interference is significant when the cells within HetNet are deployed on the same carrier frequency. It is discussed in detail in the post cross-carrier scheduling.

A number of HetNet deployment scenarios are presented by means of CA based ICIC. A promising approach is explained below by using a macro-pico example. The basic idea is to split the available spectrum into two downlink CCs denoted as CC1 (f1) and CC2 (f2), both CCs are available in both Macro and Pico layers. Macro layer configures PCell on f1 and SCell on f2 whereas, pico cell configures PCell on f2 and SCell on f1.

As shown in the figure above, three regions are of interest for control signalling (PCFICH, PDCCH, and PHICH); macro cell-center region, pico cell’s CRE region, and pico cell-center region. In the macro cell-center region, both f1 and f2 can carry control signalling. In the CRE region, macro-cell wouldn’t transmit control signalling on f2 i.e., scheduling assignments for SCell are carried by PCell on f1. So, interference caused by control signalling is minimized on f2 in CRE region.

Now, let us look at how pico cell is transmitting. Similar to macro cell, in the pico cell-center region, control signalling is transmitted on both f1 and f2. In the pico cell’s CRE region, pico cell would be transmitting control signalling only on f2 (PCell) and no transmission of control signalling on f1. i.e., scheduling assignments for SCell are carried by PCell on f2. This minimizes the interference in CRE zone which is caused by control signalling from pico cell on f1.

The downside with cross-carrier scheduling is that it will increase the load in the control region of the cell that is scheduling for another cell. This is due to the fact that the scheduling cell has to accommodate resource allocation for PDCCH for both PCell and SCell. In Release-11, a new channel known as Enhanced Physical Downlink Control Channel (EPDCCH) is introduced. This increases the control channel capacity as EDPCCH uses the same resources as PDSCH instead of control region. The EPDCCH could be used for resource allocation within each SCell without using cross-carrier scheduling. Moreover, by applying frequency domain ICIC, the reliability of receiving EPDCCH could be increased as in the case of PDSCH.

So far, the discussion was about control channels only. For data channel (PDSCH) both carriers (f1 and f2) are available in all three regions discussed above. The interference between macro and pico layers is handled by conventional ICIC method which is based on X2 signaling of RNTP between macro and pico eNBs as discussed in the post ICIC.